Fyzikální ústav Akademie věd ČR

Selected results of department 23

Free energy of mean-field spin-glass models: Evolution operator and perturbation expansion

We introduce a perturbation expansion for the evolution operator used to represent free energy with continuous replica-symmetry breaking in mean-field spin-glas models. We obtain a generic scheme for an approximate calculation of physical quantities of different mean-field spin-glass models at all temperatures. The first correction with a continuous order-parameter function beyond the approximation with one level of replica-symmetry breaking is explicitly evaluated in the glassy phase of the Sherrington-Kirkpatrick model.

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Strong-coupling d-wave superconductivity in PuCoGa5 probed by point-contact spectroscopy

We report on point-contact spectroscopy measurements in PuCoGa5, a moderate heavy-fermion superconductor with a record high critical temperature Tc = 18.5 K. The results prove that the wavefunction of the paired electrons has a d-wave symmetry, with four lobes and nodes, and show that the pairing is likely to be mediated by spin fluctuations. Electronic structure calculations, which take into account the full structure of the f-orbital multiplets of Pu, provide a hint of the possible origin of these fluctuations.

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Strong-coupling solution of the bosonic dynamical mean-field theory

We derive an approximate analytical solution of the self-consistency equations of the bosonic dynamical mean-field theory (B-DMFT) in the strong-coupling limit. The approach is based on a linked-cluster expansion in the hybridization function of normal bosons around the atomic limit. The solution is used to compute the phase diagram of the bosonic Hubbard model for different lattices. We compare our results with numerical solutions of the B-DMFT equations and numerically exact methods, respectively.  The full text >>

Resistance of ferromagnetic materials induced by spin disorder

The resistivity of ferromagnetic metals depends strongly on electron scattering by spin disorder caused by finite temperature. We have developed a method for calculation of this part of resistivity from first principles, i.e., without adjustable parameters. Our approach is based on the representation of the system by a set of disordered magnetic moments (the so-called disordered local moment method, DLM).  The full text >>

Applications of quantum Monte Carlo methods in condensed systems

Quantum Monte Carlo methods represent a powerful and broadly applicable computational tool for finding very accurate solutions of the stationary Schrödinger equation for atoms, molecules, solids and a variety of model systems. The algorithms are intrinsically parallel and are able to take full advantage of present-day high-performance computing systems. This review paper concentrates on the fixed-node/fixed-phase diffusion Monte Carlo method with emphasis on its applications to the electronic structure of solids and other extended many-particle systems.

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Monotonic return to steady nonequilibrium

We analyze how to construct Lyapunov functions describing relaxation to the stationary regime of thermodynamically open systems. These functions are time-monotonous and they can conveniently be applied to the study of stability of stationary states - examples are the free energy in equilibrium respectively the entropy production in weakly non-equilibrium systems. The main result is a proposal of a new Lyapunov function for certain far from equilibrium models, via the dynamical theory of fluctuations.  The full text >>

Demonstration of molecular beam epitaxy and a semiconducting band structure for I-Mn-V compounds

Based on density functional calculations of electronic structure and magnetic ordering we proposed that LiMnAs and related I-Mn-V crystals are antiferromagnetic semiconductors with bandgap around 1 eV. The antiferromagnetic state of these compouns is not sensitive even to high level of doping and the estimated Neel temperature is well well above the room temperature. This, together with a remarkable calculated magnetic anisotropy, makes these materials attractive for spintronics applications.  The full text >>

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